1. Field of the Invention
The present invention relates in general to a flat antenna, and more particularly to a flat antenna of the dual feeding type wherein a band width of a reception frequency can be widened and a reception efficiency can be increased.
2. Description of the Prior Art
The exchange of television programs is established between countries by interconnecting two positions far apart from each other on the earth with an electromagnetic wave by means of an artificial satellite with a transponder which is made up of a transmitter and & receiver, which is called a communication satellite. Development of the industrial technology is followed by the trend of miniaturation, lightness and thinness of products. According to such trend, there has actively been progressed the study of an antenna which is an equipment for transmission and reception of a broadcasting signal, particularly in a satellite broadcasting field.
A flat antenna is utilized as an antenna for a moving object such as satellite, airplane and the like or for reception of a satellite broadcasting signal over a frequency region from an ultra high frequency (UHF) band to a super high Frequency (SHF) band.
A typical form of an antenna for generating a circular polarization employing a micro strip antenna (MSA) for a linear polarization in the flat antennas For reception of the satellite broadcasting signal will be described hereinafter with reference to FIGS. 1 to 4. The flat antenna basically has a dielectric substrate and conductors formed on the opposite surfaces of the dielectric substrate. Namely, as shown in FIG. 1, the flat antenna comprises a dielectric substrate 1, a ground substrate 2 formed on the lower surface of the dielectric substrate 1 and a plurality of patch units (radiation elements) 3 of desired size formed on the upper surface of the dielectric substrate 1. Herein, the patch unit 3 has a length smaller than or equal to .lambda.g/2 of a useful frequency of the flat antenna.
As shown in FIG. 2, the patch units 3 are connected to one another through transformers T1-T5 and feeding lines A0-A6, thereby resulting in a provision of a feeding network. In this drawing, there is shown an example of 4*4 array flat antenna.
The main feeding line A0 is branched out into the feeding lines A1 and A2 and the transformer T1 having a length of .lambda.g/4 is provided for impedance matching at the branch point. The feeding line A1 is branched out into the feeding lines A3 and A4 and the transformer T2 having a length of .lambda.g/4 is provided for impedance matching at the branch point. Also, the feeding line A3 is branched out into the feeding lines A5 and A8 and the transformer T3 having a length of .lambda.g/4 is provided for impedance matching at the branch point. The remaining transformers T4 and T5 are provided in the same manner.
The patch units 3 constructed as mentioned above each has a diagonal slot 4 formed for the circular polarization as shown in FIG. 3, which is a detailed diagram of a portion H in FIG. 2. The diagonal slot 4 is arranged at an angle of .+-.45.degree. with respect to the feeding line A. The space between center lines of the adjacent patch units 3 is 0.7-1.0.lambda..alpha.. Assuming that a diagonal length of the diagonal slot 4 is 1 and a width thereof is wo, a reception level of the circular polarization is varied according to 1/wo.
Since a phase difference of 90.degree. is present in orthogonal mode in the flat antenna as shown in FIG. 4, a right handed circular polarization is generated when the diagonal slot 4 of the patch unit 3 is arranged at an angle of +45.degree. with respect to the feeding line A and a left handed circular polarization is generated when the diagonal slot 4 of the patch unit 3 is arranged at an angle of -45.degree. with respect to the feeding line A. For the purpose of impedance matching with a transmission circuit, the transformers T1-T5 each has a value of Zin.Zo, where Zin and Zo are input and output impedances of the patch unit 3, respectively. In other words, the transformers T1-T5 each has a value of Zo/2 to provide a feeding power uniformly and several hundred or more patch units 3 may be provided in making the flat antenna.
In such planar antenna, there may be provided a multi-stage feeding network and electromagnetic waves radiated from the respective patch units 3 are entirely in phase in view of a far electromagnetic field. As a result, such flat antenna is utilized as a directional antenna with acuteness in a particular direction.
However, the conventional flat antenna has a disadvantage, in that a frequency characteristic thereof is provided as a narrow band as shown in FIG. 5 since the slot 4 of the patch unit 3 has a small axial ratio, resulting in a low flexibility in use. Particularly when the satellite broadcasting is to be performed between the first region of about 800 MHz and the third region of about 500 MHz, the reception of the widened band signal cannot be covered with the narrow frequency band. For this reason, the construction of the flat antenna for reception of the satellite broadcasting signal is considerably difficult to embody in practice. Furthermore, in the conventional flat antenna, the radiation element (rectangular patch) has a very narrow band resulting from the use of a single feeding manner, a mutual coupling occurs between the radiation elements and the feeding network and the feeding network is exposed over the substrate, resulting in an increase loss in reception.